The advancement of the laser is remarkable in recent years, in particular, high output and compact size solid state lasers and semiconductor lasers having an emission area in near infrared to infrared regions are easily available. These lasers are very useful as exposure light sources for directly plate-making from digital data such as the computer.
The materials for positive lithographic printing plates for infrared lasers comprise, as essential components, a binder resin which is soluble in an alkali aqueous solution and an IR dye and the like which absorbs light and generates heat. The IR dye and the like functions as a dissolution inhibitor to substantially lower the solubility of the binder resin by the interaction with the binder resin at a non-exposed area (an image area), and the interaction of the IR dye, etc., with the binder resin is weakened by the heat generated at an exposed area (a non-image area), and the binder resin is dissolved in an alkali developing solution, thereby a lithographic printing plate is formed.
However, in these materials for positive lithographic printing plates for infrared lasers, the difference between the solubility resistance of a non-exposed area (an image area) in a developing solution and the solubility of an exposed area (a non-image area) in various use conditions cannot not be said to be sufficient, so that there is a problem that over-development and development failure are liable to occur due to variation in use conditions. Further, when the surface condition slightly varies by, e.g., mere touching of the surface during handling, a non-exposed area (the image area) is dissolved at development resulting in a scratch-like trace, which causes problems of the degradation in press life and ink adhesion failure.
Such problems originate in the fundamental difference in the plate-making mechanism between the positive lithographic printing plate materials for infrared lasers and the positive lithographic printing plate materials which are subjected to UV exposure for plate-making. That is, the positive lithographic printing plate materials which are subjected to UV exposure for plate-making comprise a binder resin soluble in an alkali aqueous solution, an onium salt and quinonediazide compounds as essential components, and these onium salt and quinonediazide compounds have two roles of not only functioning as a dissolution inhibitor by the interaction with the binder resin at a non-exposed area (an image area) but also being decomposed by light at an exposed area (a non-image area), to thereby generate an acid and function as a dissolution accelerator.
On the other hand, an IR dye and the like in the positive lithographic printing plate materials for infrared lasers function only as dissolution inhibitors at a non-exposed area (an image area) and do not accelerate dissolution at an exposed area (a non-image area). Accordingly, those having high solubility in an alkali developing solution cannot help being used beforehand as the binder resins for making discrimination between the solubilities at a non-exposed area and at an exposed area in the positive lithographic printing plate materials for infrared lasers. Therefore, the condition before development becomes labile.
Further, there is a problem in the positive lithographic printing plate materials for infrared lasers that sensitivity is liable to fluctuate due to a trace difference in exposure amount by the influence of a temperature increase and diffusion.